Same thing happened to me a while back when I picked up some Green Chromis. Bought 7 bright green ones, 4 got dark purple spots and died in a day or too. I didn’t see any fighting but that’s what I suspect

I have used Ruby Reef products, Prazi, and Microbe-lift Herbtana. No coral deaths for me but IMO i wouldn't buy any of these products again as i have not had any success, but that's just me.
I'm setting up a treatment tank and going to run copper, Petco is having there dollar per gallon sale right now if your looking for a cheap tank